Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The present invention relates to a method for
producing a cryostatic stabilizer, composed of high-
purity aluminum and used at ultra low temperatures.
BACKGROUND OF THE INVENTION
In those facilities and equipment whichutilize a superconductor, a conductor, generally called
a cryostatic stabilizer, is provided on and around the
superconductor to protect the superconductor by by-
passing the electric current to the aluminum conductor
around the superconductor region in the state of normal
conductivity which occurs due to an external thermal,
electric or magnetic disturbance.
DESCRIPTION OF THE RELATED ART
High-purity aluminum, because its electric
resistivity is l -rk~hly low at ultra low temperature
and in magnetic field, has been discussed for possible
use as such cryostatic stabilizer (Phys. Rev. B. Vol. 3,
No. 6, 1971, p. 1941).
As a part of such trials, the use of the
cryostatic stabilizer made of high-purity aluminum is
planned for superconducting magnetic energy storage
devices.
The cryostatic stabilizer employed in SMES
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(superconducting magnetic energy storage system) is used
as a conductor composed of a superconductor and the
cryostatic stabilizer by fixing the superconductor to
the cryostatic stabilizer with soldering or the like.
In order to uniformly keep the whole ~u~r~u--ductor at
ultra low temperatures, liquid helium needs to be
sufficiently fed to the periphery of the superconductor.
A cryostatic stabilizer having helical grooves or
projections is devised as a structure therefor [IEEE
TRANSACTIONS ON APPLIED SUPERCON~u~llvllY, Vol. 3, No.
1, p. 320 (1993)].
A method which comprises for giving a twist to
a cylindrical bar having straight grooves or projections
at its both ends to provide helical grooves are known
for obtaining the cryostatic stabilizer, composed of
high-purity aluminum and having the helical grooves or
projections.
A cryostatic stabilizer having helical grooves
or projections at a uniform pitch throughout the whole,
however, is not readily obtained according to the method
mentioned above, and properties satisfactory for the
cryostatic stabilizer cannot be directly accomplished by
such method because electric resistance of the obtained
cryostatic stabilizer at ultra low temperatures becomes
too large for the practical use especially when the
cross-sectional area of the cryostatic stabilizer is
relatively small. Therefore, for lowering the electric
resistivity at ultra low temperatures, it is necessary
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further to carry out the heat treatment after the method
mentioned above, and a ,. rkAhle increase in cost
cannot be avoided.
The object of the present invention is to
provide an industrially advantageous method for
producing a cryostatic stabilizer, composed of high-
purity aluminum and having a low electric resistance at
ultra low temperatures and helical grooves or projec-
tions without requiring any heat treatments. This and
other objects and advantages will be apparent from the
following description.
SUMMARY OF THE INVENTION
As a result of intensive research made on the
method for producing the cryostatic stabilizer composed
of high-purity aluminum, the present inventors have
found that a cryostatic stabilizer having a low electric
resistance at ultra low temperatures is advantageously
obtained by extruding the high-purity aluminum under
specific conditions using an extruding machine equipped
with a die of a specified shape, and the present
invention has been completed.
Thus, this invention relates to a method for
producing a cryostatic stabilizer composed of high-
purity aluminum which comprises extruding the high-
purity aluminum at an extrusion temperature of 250 to500~C, an extrusion speed of not more than 20 m/min and
an extrusion ratio of 10 to 150 by using an extruding
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machine equipped with a cylindrical die having helical
grooves or projections on the inner surface.
This invention is illustrated hereinafter.
The high-purity aluminum used in this
invention has a purity of at least 99.9% by weight.
When aluminum having lower purity than 99.9% by weight
is used, its electric resistance at ultra low tempera-
ture can't be lowered to such an extent as to be usable
for the cryostatic stabilizer. Therefore, such aluminum
having lower purity than 99.9% by weight is unsuitable
as the cryostatic stabilizer. The purity of the high-
purity aluminum is usually 99.9 to 99.9999% by weight,
preferably 99.99 to 99.9999% by weight from aspects of
industrial production and performances as the cryostatic
stabilizer.
In the present invention, the purity of the
high-purity aluminum means weight % obtained by
deducting, from 100, weight % of metallic and semi-
metallic elements other than aluminum which are detected
by, for example, GDMS (Glow Discharge Mass
Spectroscopy).
The extruding machine used herein is equipped
with, for example, a cylindrical die having helical
grooves or projections on the inner surface thereof.
The pitch of the helical grooves or
projections of the die is usually 5 inches/l turn to 50
inches/l turn. The pitch of the helical grooves or
projections thereof is sufficiently at the pitch of 5
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inches/l turn from the viewpoint that the cryostatic
stabilizer effectively acts on cooling. When the pitch
is smaller than 5 inches/1 turn, the die is not
advantageous to its productivity. When the pitch is
larger than 50 inches/l turn, there is little difference
from that of straight grooves or projections in cooling
efficiency.
The number of the grooves or projections of
the die, the width of the projections or distance
between the adjacent grooves, the height of the
projections, the depth of the grooves, shape of the
grooves or projections or the like can be suitably
determined according to the shape of the applied SMES
conductor. The number of the grooves or projections of
the die is usually 2 to 100, preferably 4 to 100. Since
the diameter of the superconductor used for the
conductor is usually about 1 to 10 mm, the width of the
projections or the distance between the adjacent grooves
is usually about 1.1 times or above that of the
superconductor. The height of the projections or depth
of the grooves of the die is usually about 1 to 10 mm.
The diameter (maximum diameter) of the die
opening can be suitably det~rmi ne~ according to the
electric current applied in the stabilizer. The
cryostatic stabilizer according to the method of the
present invention can be applied even to the one having
a diameter of about 100 mm or below which cannot be used
without heat treatment in conventional method. Thus,
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this method is especially useful for producing the
cryostatic stabilizer having a diameter of about 100 mm
or below.
As the extrusion method with the extruding
S machine, for example, direct, indirect or hydraulic
extrusion methods, continuous extrusion forming methods
or the like can be applied. Among them, the direct
extrusion method is preferred.
The cryostatic stabilizer, composed of the
high-purity aluminum and having a shape corresponding
to, for example, a cylindrical die having helical
grooves or projections on the inner surface is obtained
by using an extruding machine equipped with the die as
mentioned above.
The extrusion ratio [cross-sectional area of
upset ingot/cross-sectional area of extrusion] in the
present invention is lO to 150. if the cryostatic
stabilizer is produced at an extrusion ratio below 10,
the cryostatic stabilizer having uniform electric
resistance at ultra low temperatures can~t be obtained.
If the extrusion ratio exceeds 150, the helical grooves
or projections of the cryostatic stabilizer are not
sufficiently produced. The extrusion ratio is
preferably 20 to 100.
The extrusion temperature in the present
invention is 250 to 500~C. If the temperature is below
250~C, the electric resistance of the cryostatic
stabilizer at ultra low temperatures is too large, and a
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satisfactory cryostatic stabilizer can't be obtained
without heat treatment. If the extrusion temperature
exceeds 500~C, the stiffness of the material is lowered,
and helical projections or grooves having the objective
pitch cannot be formed. The extrusion temperature is
preferably 300 to 450~C.
The extrusion speed in the present invention
is not more than 20 m/min. If the extrusion speed
exceeds 20 m/min, cracking occurs, and a satisfactory
shape of the cryostatic stabilizer is not obtained. The
optimum speed according to the objective pitch can be
suitably selected, and the extrusion speed is usually
0.1 to 20 m/min, preferably 0.2 to 10 m/min in its
productivity.
Since the cryostatic stabilizer extruded from
the outlet of the extruding machine can be extruded
rotationally according to the pitch of the die, the
cryostatic stabilizer is preferably led out according to
the rotational pitch of the extrusion and the leading
out is effective in uniformizing the pitch of the
helical projections or grooves of the resulting
cryostatic stabilizer. The leading out may be carried
out by selecting a proper speed according to the pitch
of the helical projections or grooves.
The residual resistivity ratio of the
cryostatic stabilizer in the present invention is a
value represented by A/B when the electric resistance of
a sample bar having a diameter of 25.4 mm and a length
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of 150 mm at room temperature (296 K) is A and the
electric resistance thereof at the ultra low temperature
(4-2 K) is B.
A sample having a diameter of 25.4 mm and a
length of 150 mm was heat-treated at 500~C in the air
for 3 hours and then was returned to the room tempera-
ture over a period of 24 hours. The electric resistance
of the resultant sample at 296 K is A' and the electric
resistance at 4.2 K is B'. The residual resistivity
ratio of the raw material is a value represented by
A'/B'.
It is preferable that the cryostatic
stabilizer, composed of the high-purity aluminum and
having helical projections or grooves maintains the
residual resistivity ratio of the raw material as it is;
however, it is unavoidable that the residual resistivity
ratio is lowered by the strain produced in extrusion
working.
The cryostatic stabilizer composed of the
high-purity aluminum according to the present invention
has a residual resistivity ratio (A/B) of 50% or above
based on that of the raw material (A'/B'), and provide a
sufficiently permissible electric resistance for
practical use at ultra low temperatures. If the
residual resistivity ratio (A/B) is below 50~ based on
that of the raw material (A'/B'), the electric resist-
ance at ultra low temperatures is too large for
practical use.
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According to the present invention, the
excellent cryostatic stabilizer, composed of the high-
purity aluminum and having a low electric resistance at
ultra low temperatures can be industrially and advanta-
geously obtained without requiring a heat-treating step.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
This invention is illustrated by citing the
following Examples, which are not construed as limiting
the invention.
Example 1
An extruding machine (1500-ton extruding
machine, manufactured by NIHON TEKKO, ~td.) equipped
with a cylindrical die (material: JIS SKD61) having the
opening diameter of 25.4 mm and 8 helically (pitch: 15
inches/1 turn) engraved projections [projection width: 4
mm; distance between the projections (projection
bottom): 6 mm; projection height: 4.5 mm] at a regular
interval on the inner surface was used to extrude a
billet having a diameter of 155 mm (aluminum purity:
99.9996% by weight) at an extrusion temperature of 400~C
and an extrusion speed of 0.6 m/min. Thereby, a bar of
the high-purity aluminum having an outside diameter of
25.4 mm, a purity of 99.9996% by weight and 8 helical
grooves [pitch: 15 inches/1 turn groove bottom width: 4
mm, distance between the grooves (groove top): 6 mm;
groove depth: 4.5 mm] at a regular interval was
produced, and a sample of 150 mm long was cut from the
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bar. The electric resistance of the resulting sample
was measured in liquid helium (4.2 K) and room
temperature (296 K) by a potentiometric method with a dc
comparator potentiometer (Guildline, Model 9930) to
obtain the residual resistivity ratio (electric resist-
ance at 296 K/electric resistance at 4.2 K). The
results are shown in Table 1.
Comparative Example 1
An extruding machine (1500-ton extruding
machine, manufactured by NI~ON TEKKO, Ltd.) equipped
with a cylindrical die having the opening diameter of
25.4 mm and 8 straight engraved grooves [projection
width: 4 mm, distance between the projections
(projection bottom): 6 mm; projection depth: 4.5 mm] on
the inner surface was used to extrude a billet having a
diameter of 155 mm (aluminum purity: 99.9996% by weight)
at an extrusion temperature of 260~C and an extrusion
speed of 15 m/min. Thereby, a bar of the high-purity
aluminum having an outside diameter of 25.4 mm and a
purity of 99.9996% by weight and 8 straight grooves
[groove top width: 4 mm; distance between the grooves
(groove top): 6 mm; groove depth: 4.5 mm] at a regular
interval was produced, and a bar having a groove pitch
of 15 inches/1 turn was obtained by giving a twist to
the bar at both ends. A sample of 150 mm long was cut
from the bar, which was measured in the same manner as
in Example 1. The results are shown in Table 1.
~able 1
Raw materia_ Grooved bar
Electric Electric Electric Electric
Resistance Resistance Residual Resistance Resistance Residual
at 296 K at 4.2 K resistivity at 296 K at 4.2 Kresistivity
ratio ratio
(n~) (nQ) (nQ) (nQ)
Example 17770 1.04 7471 8250 1.53 5392
Compara- 7770 1.04 7471 5060 3.41 1483
tive
Example 1
O;~
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As can be seen from Table l, the residual
resistivity ratio in using the cylindrical die having
the helically engraved grooves on the inner surface
(Example 1) was 3.6 times (5392 . 1483) of that in using
the cylindrical die having the rectilinearly engraved
grooves (Comparative Example 1). In Example 1, the
residual resistivity ratio was 72% (5392 . 7471 x 100)
based on that of the raw material. On the other hand,
the residual resistivity ratio in Comparative Example 1
was 20~ (1483 . 7471 x 100) based on that of the raw
material.
